Evolution is the fundamental theory of biology but is frequently found difficult for learners. Research on evolution understanding has focused concepts such as variation, selection and inheritance. Also more abstract concepts (such as time scales, spatial scales, randomness, probability) can act as thresholds to understand evolution. We have shown that educational visualizations often lack these, possibly due to an effort to reduce complexity and problems in how to visualize these concepts. We aim to assess whether using threshold concepts as a design heuristic facilitates understanding of evolution of antibiotic resistance. The thresholds are addressed in the design in the following manner: randomness is visualized as accidental incorporation of mismatched nucleotides in DNA-replication, spatial scales as zoom-in sequences from macro-scale (test-tube) to molecular level (DNA), time scales as number of generations, and probability as population numbers compared with the probability of single beneficial mutations. Data from user testing is currently under analysis and tentative results will be presented. The results will also inform further development of the visualization.